This invention relates to high-activity silicoaluminophosphates (SAPOs) having unique silicon distributions, a method for their preparation and their use as Fluid Catalytic Cracking (xe2x80x9cFCCxe2x80x9d) catalysts. More particularly, the new SAPOs, designated herein as ECR-42, have a high silica:alumina ratio and a beneficial silicon atom distribution.
The need for low emissions fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production.
Fluidized and fixed bed processes have recently attracted renewed interest for increasing olefin production. For example, U.S. Pat. No. 4,830,728 discloses an FCC unit that is operated to maximize olefin production. The FCC unit has two separate risers into which a different teed stream is introduced. The operation of the risers is designed so that a suitable catalyst will act to convert a heavy gas oil in one riser and another suitable catalyst will act to crack a lighter olefin/naphtha feed in the other riser. Conditions within the heavy gas oil riser can be modified to maximize either gasoline or olefin production. The primary means of maximizing production of the desired product is by using a specified catalyst.
Often, conventional hydrocarbon conversion processes utilize crystalline zeolites as catalysts. Crystalline zeolites have been found to be effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce naphthas and olefins. Conventionally, crystalline zeolite is incorporated into a matrix in order to form a catalytic cracking catalyst.
It is also known that improved cracking results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 xc3x85 is included with a crystalline zeolite having a pore size greater than 8 xc3x85, either with or without a matrix, as set forth in U.S. Pat. No. 3,769,202. In this regard, a large pore size crystalline zeolite (pore size greater than 7 xc3x85) may be used in admixture with ZSM-5 type zeolite. The use of ZSM-5 in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934.
There remains a need, though, for improved catalysts that may be added to conventional (i.e., large pore) FCC catalyst, especially additive catalysts that result in improved activity and selectivity for light olefin formation, especially propylene formation.
Conventional silicoaluminophoshates such as SAPO-11 are catalytically active molecular sieves, especially at high silicon concentrations. However, such materials are prone to detrimental silicon distribution in the framework at high silicon concentration, above about 0.04 molar Si fraction in the framework, resulting in a diminished catalytic activity. Moreover, even materials with a favorable Si distribution in the framework may undergo an undesirable redistribution of framework Si during, for example, catalyst regeneration. There is therefore a need for improved silicoaluminophoshates that have increased catalytic activity at high silicon concentration, and improved activity maintenance characteristics.
In one embodiment, the invention is a composition comprising a major amount of a large-pore zeolite catalyst and a minor amount of a molecular sieve composition with a framework tetrahedra of silicon, aluminum, and phosphorus, the molecular sieve having a total silicon amount ranging from above about 4 molar percent to about 20 molar percent, a total aluminum amount ranging from about 40 molar percent to about 55 molar percent, and a total phosphorus amount ranging from about 30 molar percent to about 50 molar percent, the molar percents being based on the total amount of aluminum, phosphorus, and silicon present in the composition, and the molecular sieve having the topology AEL and being isostructural with conventional SAPO-11, wherein
(a) the silicon present in the molecular sieve and the conventional SAPO-11 is distributed among silicon sites in the framework tetrahedra, each site having a first, a second, a third, and a fourth nearest neighbor position, and each nearest neighbor position being independently occupied by one atom selected from silicon, and aluminum, and
(b) the molecular sieve has a substantially smaller number of silicon sites having silicon atoms among all four nearest neighbor positions than the conventional SAPO-11 with the total silicon amount.
In another embodiment, the invention is a fluidized catalytic cracking method comprising injecting a feed into an FCC riser reactor having a reaction zone and catalytically cracking the feed in the reaction zone under catalytic cracking conditions in the presence of a catalytically effective amount of a cracking catalyst in order to form a cracked product, the cracking catalyst containing a molecular sieve catalyst composition with a framework tetrahedra of silicon, aluminum, and phosphorus, the molecular sieve having a total silicon amount ranging from above about 4 molar percent to about 20 molar percent, a total aluminum amount ranging from about 40 molar percent to about 55 molar percent, and a total phosphorus amount ranging from about 30 molar percent to about 50 molar percent, the molar percents being based on the total amount of aluminum, phosphorus, and silicon present in the composition, and the molecular sieve having the topology AEL and being isostructural with conventional SAPO-11, wherein
(a) the silicon present in the molecular sieve and the conventional SAPO-11 is distributed among silicon sites in the framework tetrahedra, each site having a first, a second, a third, and a fourth nearest neighbor position, and each nearest neighbor position being independently occupied by one atom selected from silicon, and aluminum, and
(b) the molecular sieve has a substantially smaller number of silicon sites having silicon atoms among all four nearest neighbor positions than the conventional SAPO-11 having the same total silicon amount.
In another embodiment, the invention is a fluidized catalytic cracking method comprising injecting a feed into an FCC riser reactor having a reaction zone and catalytically cracking the feed in the reaction zone under catalytic cracking conditions in the presence of a cracking catalyst in order to form a cracked product, the cracking catalyst containing a major amount of a large-pore zeolite catalyst and a minor amount of a molecular sieve catalyst with a framework tetrahedra of silicon, aluminum, and phosphorus, the molecular sieve having a total silicon amount ranging from above about 4 molar percent to about 20 molar percent, a total aluminum amount ranging from about 40 molar percent to about 55 molar percent, and a total phosphorus amount ranging from about 30 molar percent to about 50 molar percent, the molar percents being based on the total amount of aluminum, phosphorus, and silicon present in the composition, and the molecular sieve having the topology AEL and being isostructural with conventional SAPO-11, wherein
(a) the silicon present in the molecular sieve and the conventional SAPO-11 is distributed among silicon sites in the framework tetrahedra, each site having a first, a second, a third, and a fourth nearest neighbor position, and each nearest neighbor position being independently occupied by one atom selected from silicon, and aluminum, and
(b) the SAPO molecular sieve has a substantially smaller number of silicon sites having silicon atoms among all four nearest neighbor positions than the conventional SAPO-11 having the same total silicon amount.
In another embodiment, the invention is a composition comprising a major amount of a large-pore zeolite catalyst and a minor amount of a molecular sieve, the molecular sieve having a framework tetrahedra of silicon, aluminum, and phosphorus, a total silicon amount ranging from above about 4 molar percent to about 20 molar percent, a total aluminum amount ranging from about 40 molar percent to about 55 molar percent, and a total phosphorus amount ranging from about 30 molar percent to about 50 molar percent, the molar percents being based on the total amount of aluminum, phosphorus, and silicon present in the composition, and the molecular sieve having the topology AEL and being isostructural with conventional SAPO-11, wherein
(a) the molecular sieve has a first number of Si atoms coordinated as Si(4Si),
(b) the conventional SAPO-11 with the Si amount has a second number of Si atoms coordinated as Si(4Si), and
(c) the first number of Si atoms is substantially less than the second number of Si atoms.
In another embodiment, the invention is a fluidized catalytic cracking method comprising injecting a feed into an FCC riser reactor having a reaction zone and catalytically cracking the feed in the reaction zone under catalytic cracking conditions in the presence of a cracking catalyst in order to form a cracked product, the cracking catalyst containing a molecular sieve having a framework tetrahedra of silicon, aluminum, and phosphorus, a total silicon amount ranging from above about 4 molar percent to about 20 molar percent, a total aluminum amount ranging from about 40 molar percent to about 55 molar percent, and a total phosphorus amount ranging from about 30 molar percent to about 50 molar percent, the molar percents being based on the total amount of aluminum, phosphorus, and silicon present in the composition, and the molecular sieve having the topology AEL and being isostructural with conventional SAPO-11, wherein
(a) the molecular sieve has a first number of Si atoms coordinated as Si(4Si),
(b) the conventional SAPO-11 with the Si amount has a second number of Si atoms coordinated as Si(4Si), and
(c) the first number of Si atoms is substantially less than the second number of Si atoms.